NFB Loop Help !

[Cejay825]

What exactly does swapping these wires do..... like, does it do anything to the sound of the amp or change the tone in any fashion ? What would the amp sound like in stock form with these wires swapped ? Any how, I suppose you unsolder them from the power tube pins and switch them ? Thanks for the help guys

[Colossal]

I should add that in the short amount of time I've been on this forum, Martin has consistently shown he's at the top of his game. I tend to take his advice as gospel.

+1

[Cejay825]

JAZZ, you may be on to something, after doing some preliminary reading on this, ....the blue and brown wires correct impedance are not relevant UNLESS you have a NFB loop applied (so says the internet)........will I have to do anything with my speaker wiring configuration after switching OT secondaries ?

[johnnyreece]

Just to ensure you're reading it right, be sure you're switching the PRIMARIES and not the SECONDARIES. It's always a crapshoot for me wiring in an OT with NFB. I do tend to be lucky more often than not and get it the first time, but basically, if you have them in backwards, you get positive feedback (which is heard as a squeal). If that's all you switch, there shouldn't be any other changes necessary (unless the squeal gets worse, then switch it back and go back to the drawing board). Good luck!

[Cejay825]

Just to ensure you're reading it right, be sure you're switching the PRIMARIES and not the SECONDARIES. It's always a crapshoot for me wiring in an OT with NFB. I do tend to be lucky more often than not and get it the first time, but basically, if you have them in backwards, you get positive feedback (which is heard as a squeal). If that's all you switch, there shouldn't be any other changes necessary (unless the squeal gets worse, then switch it back and go back to the drawing board). Good luck!

lol Not sure of the terminology but I do know the blue and brown wires to power tubes are the ones to switch.....Man I'm a rookie !

[johnnyreece]

No worries. I'm not much better. Just wanted to give you a heads-up!

[martin manning]

...will I have to do anything with my speaker wiring configuration after switching OT secondaries (sic)?

When you switch the primary (blue and brown) OT leads the signal at the speaker will be inverted. That means that whatever phase relationship it had originally from the input to the speaker will be flipped. Some people claim that this matters. If there is an effect I think it will be due to the mechanical feedback from the speaker to the chassis or feedback via the varying magnettic field generated by the speaker. If you find that the squeal goes away with the primaries reversed, then you might want to reverse the connections at the speaker and see if it makes any difference.

[Cejay825]

Sounds good, I'll give it a try when I get home this evening. Thanks everyone

[Cejay825]

Well, great news, switching the wires worked !!!! and with out having to rewire the speaker leads, I guess they were unaffected. I still have some questions though. When I turn the pot clock wise, to its lowest ohm reading, there's a tiny bite of travel right near the end where the volume will reduce dramatically. Maybe this is something to do with the resistor values ?? Also I would like to install a switch to turn the loop off and revert back to the 18k tail resistor .....where should it be in the circuit or is this possible. Thanks a million to you guys !!!!

[Cejay825]

Ahhhhhh, I see, the 13k3 and the 4k7 make up the 18k when NFB loop is at a minimum.....I'm learning Martin..... and maybe the drop off in volume is where the NFB loop is turning off, like JAZZ said..... I think I've made sense out of it. So really all i need to do is interrupt the wire leading from the pot going to the tail resistor with a switch ????

[JazzGuitarGimp]

You can place the switch anywhere in the pot circuit; Between the pot and the FB resistor; Between the FB resistor and the output transformer; or between the pot and the left end of R48 (in this case, just be sure to leave the connection between R48 and the new 4K7 resistor.

Glad to hear you've got it working. Great work!

[FUCHSAUDIO]

if the amp is louder with the loop connected, you may have (or I should say you LIKELY have) positive feedback. OT primary lead swap should fix it. While there are all kinds of measurements you can take (if you have the test gear), you could do it by ear really, or use existing values in other similar Fender designs.